When you press Ctrl+alt+del you then have to choose to open Task Manager from a list of other options. Ctrl+shift+esc skips that step and just opens Task Manager directly. Try it.
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JD IsaacksJan 13 '12 at 19:44

5 Answers
5

System Monitor shows you an overview of running applications (under the Processes tab) and allows you to end them by right-clicking on the name and selecting the respective context menu item. You can set up keyboard shortcuts in the Keyboard settings (Shortcuts tab), so you can bind Ctrl+Alt+Del to gnome-system-monitor, the command to start System Monitor.

Another command you might be interested in is xkill. This is traditionally bound to Ctrl+Alt+Esc (not sure if by default) and allows you to click on a misbehaving window to close it.

Both of these might not work if your entire system hangs. If that happens, there are two things you can do. The first has to be enabled in advance when your system doesn't hang yet (you could do it right now): open Keyboard Layout settings (I believe this is merged into Keyboard in Ubuntu 12.04, but that hasn't been released yet), then click Options. One of the options is Key sequence to kill the X server, you can click that to enable Ctrl+Alt+Backspace. If you have done that, and your system hangs, you can press Ctrl+Alt+Backspace, which will effectively bring you back to the login screen.

If even that doesn't work, the last thing you can do is ensuring a sane shutdown (i.e. not pressing and holding the power button). This one is a bit hard to remember, but it involves pressing and holding Alt+PrtSc and then press in order R, E, I, S, U, B (a mnemonic is Reboot Even If System Utterly Broken).

How do I know what is causing the system hang? In windows, task manager shows which application is NOT RESPONDING from which the user can end the process controlling the application in question
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MysterioJan 13 '12 at 18:04

Oh, usually for me the windows of an application that no longer responds becomes a lot darker. You can enable this in CompizConfig Settings Manager (you might need to install this, the package name is ccsm). Click the Fading Windows plugin, and there you can check "Dim Unresponsive Windows". Then in the future you should know which window to kill.
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VincentJan 14 '12 at 13:38

ctrl+alt+backspace combination (disabled on default on 11.10) that can restart the GUI.

ctrl+alt+F2 to F6, that will display a console from which you could login and then eventually kill the stucked application. Once you killed it you can return to the GUI by pressing alt-f7. Killing an application by name can be done by using this command:

sudo killall <name-of-the-application>

If this doesn't work, use the -9 flag to kill it with überforce.

sudo killall -9 <name-of-the-application>

IN CASE OF EMERGENCY : use the Magic SysReq Key to directly "speak" to the kernel.

+1 sysrq. why noone mentions ctrl+printscreen+K, that will restart the GUI and kill all processes in the gui (in case you didn't expect that from a restart). sysrq is the same key as printscreen key.
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naxaJul 9 '13 at 0:33

you could have a short command for opening a terminal, I have F4.
When you need to force an application to quit just open a terminal and type xkill and then click in the window of the application that has crashed.